In recent years, transistors which are used for many liquid crystal display devices and light-emitting display devices typified by flat panel displays have included a silicon semiconductor such as amorphous silicon or polycrystalline silicon and have been formed over glass substrates.
Instead of the silicon semiconductor, a technique in which an oxide semiconductor is used for transistors has attracted attention.
Examples of the oxide semiconductor include zinc oxide which is a one-component metal oxide and an In—Ga—Zn—O-based oxide semiconductor which is a homologous compound. In addition, techniques, in which transistors are formed using these oxide semiconductors and the transistors are used for switching elements and the like of pixels in display devices, are disclosed (see Patent Document 1 and Patent Document 2).